ASS1

The ASS1 gene provides instructions for making an enzyme called argininosuccinate synthetase 1. This enzyme participates in the urea cycle, which is a sequence of chemical reactions that takes place in liver cells. The urea cycle processes excess nitrogen that is generated as the body uses proteins. The excess nitrogen is used to make a compound called urea, which is excreted from the body in urine.

Argininosuccinate synthetase 1 is responsible for the third step of the urea cycle. This step combines two protein building blocks (amino acids), citrulline and aspartate, to form a molecule called argininosuccinic acid. A series of additional chemical reactions uses argininosuccinic acid to form urea.

citrullinemia - caused by mutations in the ASS1 gene

At least 50 mutations that cause type I citrullinemia have been identified in the ASS1 gene. Most of these mutations change single amino acids in the argininosuccinate synthetase 1 enzyme. These genetic changes likely alter the structure of the enzyme, impairing its ability to bind to molecules such as citrulline and aspartate. A few mutations lead to the production of an abnormally short version of the enzyme that cannot effectively play its role in the urea cycle.

Defects in argininosuccinate synthetase 1 disrupt the third step of the urea cycle, preventing the liver from processing excess nitrogen into urea. As a result, nitrogen (in the form of ammonia) and other byproducts of the urea cycle (such as citrulline) build up in the bloodstream. Ammonia is toxic, particularly to the nervous system. An accumulation of ammonia during the first few days of life leads to poor feeding, vomiting, seizures, and the other signs and symptoms of type I citrullinemia.